What a great thing you are doing! I have not worked with TNT but did do the Avon Breast Cancer 3 Day Walk with my mom this year. We only needed to raise $1900 each which I know is a lot less than TNT. We had great success with sending a letter to EVERYONE we know including local businesses---between us raised close to $6000 on this alone. A friend did TNT and said that she worked with a group in her area that was fundraising for the marathon and did larger fundraisers and then split the money raised. For example they were able to have a comedy club donate profits from one night to them. Another friend had people donate things to her and then had a huge garage sale in which she raised quite a lot of money. Lots of people have "junk" they want to get rid of that they would be happy to donate for a sale.
Anyway, best of luck and I hope you find the TNT marathon to be as wonderful an experience as I found the walk to be!